1. Field of the Invention
The present invention relates to the production of stencils for screen printing.
2. Related Background Art
The production of screen printing stencils is generally well known to those skilled in the art.
One method, referred to as the xe2x80x9cdirect methodxe2x80x9d of producing screen printing stencils involves the coating of a liquid light-sensitive emulsion directly onto a screen mesh. After drying, the entire screen is exposed to actinic light through a film positive held in contact with the coated mesh in a vacuum frame. The black portions of the positive do not allow light to penetrate to the emulsion which remains soft in those areas. In the areas which are exposed to light, the emulsion hardens and becomes insoluble, so that, after washing out with a suitable solvent, the unexposed areas allow ink to pass through onto a substrate surface during a subsequent printing process.
Another method, referred to as the xe2x80x9cdirect/indirect methodxe2x80x9d involves contacting a film, consisting of a pre-coated unsensitised emulsion on a base support, with the screen mesh by placing the screen on top of the flat film. A sensitised emulsion is then forced across the mesh from the opposite side, thus laminating the film to the screen and at the same time sensitising its emulsion. After drying, the base support is peeled off and the screen is then processed and used in the same way as in the direct method.
In the xe2x80x9cindirect methodxe2x80x9d a film base is pre-coated with a pre-sensitised emulsion. The film is exposed to actinic light through a positive held in contact with the coated film. After chemical hardening of the exposed emulsion, the unexposed emulsion is washed away. The stencil produced is then mounted on the screen mesh and used for printing as described above for the direct method.
In the xe2x80x9ccapillary direct methodxe2x80x9d a pre-coated and pre-sensitised film base is adhered to one surface of the mesh by the capillary action of water applied to the opposite surface of the mesh. After drying, the film is peeled off and the screen then processed and used as described for the direct method.
In addition to the above methods, hand-cut stencils can be used. These are produced by cutting the required stencil design into an emulsion coating on a film base support. The cut areas are removed from the base before the film is applied to the mesh. The emulsion is then softened to cause it to adhere to the mesh. After drying, the base is peeled off. The screen is then ready for printing. This method is suitable only for simple work.
One problem generally associated with all the prior art methods is that many steps are necessary to produce the screen, thus making screen production time-consuming and labour-intensive.
Another problem is that normal lighting cannot be used throughout the screen production process in any of the methods except hand cutting. This is because the stencil materials are light-sensitive. In addition, it is necessary to provide a source of actinic (usually UV) light for exposing the stencil. This usually incurs a penalty of initial cost, space utilisation and ongoing maintenance costs.
Other methods of preparing printing screens are available. CA-A-2088400 (Gerber Scientific Products, Inc.) describes a method and apparatus in which a blocking composition is ejected directly onto the screen mesh surface in a pre-programmed manner in accordance with data representative of the desired image. The blocking composition directly occludes areas of the screen mesh to define the desired stencil pattern.
EP-A-0492351 (Gerber Scientific Products, Inc.) describes a method where an unexposed light-sensitive emulsion layer is applied to a screen mesh surface and a graphic is directly ink-jet printed on the emulsion layer by means of a printing mechanism to provide a mask through which the emulsion is exposed before the screen is further processed.
Both the above methods require the use of very specialised equipment (because of the need to handle large complete screens) which incurs a certain cost as well as imposing restrictions arising from the limitations of the equipment, in particular in terms of the size of screen and its resolution.
Ink-jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink-jet printers can be used to produce a wide variety of printed materials, including text, graphics and images on a wide range of substrates. In many ink-jet printing systems, ink is printed directly onto the surface of the final receiving substrate. An ink-jet printing system where an image is printed on an intermediate image transfer surface and subsequently transferred to the final receiving substrate is disclosed in U.S. Pat. No. 4,538,156 (ATandT Teletype Corp.). Furthermore, U.S. Pat. No. 5,380,769 (Tektronix Inc.) describes reactive ink compositions containing at least two reactive components, a base ink component and a curing component, that are applied to a receiving substrate separately. The base ink component is preferably applied to the receiving substrate using ink-jet printing techniques and, upon exposure of the base ink component to the curing component, a durable, crosslinked ink is produced.
According to the present invention there is provided a method of producing a screen-printing stencil having open areas and blocked areas for respectively passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer capable of receiving a first chemical agent in areas corresponding to the blocked areas of the stencil to be produced;
applying the first chemical agent to the image-receiving layer of the receptor element in the said corresponding areas;
applying a second, stencil-forming chemical agent to a screen printing screen;
bringing the image-receiving layer of the receptor element into contact with the stencil-forming agent, to allow the first and second chemical agents to react to produce on the screen a stencil-forming layer having areas of lower solubility corresponding to the said blocked areas and areas of higher solubility in areas corresponding to the open stencil areas;
removing any remaining unreacted part of the receptor element; and
washing away the second chemical agent in the higher solubility areas, thereby to produce the screen-printing stencil.
In the method of the invention, the stencil is formed by chemical means without the need to use either special lighting conditions or actinic radiation.
Also, it is possible to carry out the method at reduced expenditure of time and time labour, compared with the known processes.
The steps of removing any remaining unreacted part of the receptor element and of washing away the second chemical agent in the higher solubility areas can be carried out in either order or simultaneously. Thus, when the unreacted part of the receptor element comprises a coherent film (for example the optional support base referred to or the image-receiving layer itself), the film can be removed, for example by being peeled away, before the washing away step. Alternatively, the film can be removed in the course of the washing away step, either by the washing action or otherwise, or even be removed after the washing away step. In some cases however the remaining unreacted part of the receptor element may be a material which is removed by the washing action, for example when the optional support base is absent and the image-receiving layer is insufficiently coherent to be removed as an intact layer, for example by peeling away.
Advantageously, the first chemical agent is applied dropwise to the image-receiving layer.
Conveniently, the dropwise application is by use of an ink-jet device, for example an ink-jet printer or plotter. The device may have one or more ejection heads.
If desired, the first chemical agent may be produced in situ by reaction between two or more precursor materials, separately applied to the image-receiving layer, prior to contact with the stencil forming agent, at least one of which is applied in the said areas corresponding to the blocked areas of the stencil to be produced. This may conveniently be achieved by use of a plurality of drop-ejection heads.
When dropwise application is employed, the application is preferably controlled according to data encoding the desired pattern of blocked and open areas of the stencil to be produced. This control is conveniently by a computer, for example a personal computer. Thus, data representative of the desired output pattern can be input to a controller as pre-recorded digital signals which are used by the ejection head to deposit or not deposit the liquid containing the chemical agent as it scans the surface of the receptor element. The invention is not however restricted to dropwise application of the first chemical agent: other methods of application will achieve the same essential end, for example, the first chemical agent could be applied with a hand-held marker pen.
The method according to the invention can be carried out using a material of the image-receiving layer which is essentially unreactive with the first chemical agent. In such a process, the image-receiving layer acts essentially as an inert carrier for the first chemical agent. The stencil-forming layer of the eventual stencil is thus derived essentially from the second chemical agent applied to the screen.
Preferably however the material of the image-receiving layer is selected to react with the first chemical agent to produce lower solubility areas corresponding to the said blocked areas and excess of the first chemical agent (or a component of it, not necessarily the same as the component that reacts with the image-receiving layer) remains in said areas to react with the second chemical agent upon contact between the image-receiving layer and the stencil-forming agent, whereby the respective lower solubility areas of the image-receiving layer and of the stencil-forming layer combine with one another and, after the higher solubility areas are washed away, remain to form the blocked areas of the screen-printing stencil.
In such a method, the stencil-forming layer of the eventual stencil is derived in part from the second chemical agent and in part from the image-receiving layer of the receptor element. In this case, the thickness of the stencil-forming layer can be such as to give the eventual screen a xe2x80x9cprofilexe2x80x9d, that is a significant thickness to the closed areas of the stencil beyond the thickness of the screen itself. This is of benefit in terms of the quality of printed images which are obtainable by use of the screen as it allows a significant ink deposit to be applied during printing and permits more precise control of the amount of ink deposited. It also produces a flat printing surface which gives better resolution and improved definition by limiting ink spread during printing.
In one variant of the method of the invention, the second chemical agent is applied to the screen printing screen from one side thereof after the receptor element has been applied to the other side thereof with its image-receiving layer in contact with the screen, whereby the image-receiving layer is brought into contact with the second chemical agent.
In another variant, the second chemical agent is applied to the screen printing screen and the receptor element is subsequently brought into contact with the screen to bring the image-receiving layer thereof into contact with the second chemical agent.